This invention relates generally to the hole cutting tool art and more particularly to rotary cutting devices for boring holes in a workpiece.
The ability to quickly, efficiently and accurately form a hole through a workpiece has many applications. As such, numerous types of hole cutting devices have been devised. Auger and spiral bits are probably the most common for drilling small diameter holes, however, larger diameter holes require a different bit structure.
For example, two general types of hole cutting devices include a serrated band formed in a circular pattern and a single edge boring device. The serrated band is formed by cutting serrations along one edge of a band of material and forming the band of material into a cylindrical shape. The cylindrical shape is mounted so that it may be retained as such under the force of rotation. Rotary movement of the cylindrical band forms a bore. A problem arises with this type of device in that the band requires many serrations to be formed therein and is limited to materials which facilitate forming a band into a cylinder. A similar device formed from a rigid cylindrical body and not a band, typically requires many machining operations in order to achieve desired results.
Another form of cutting device is a single edge boring device. An example of such a device is the UNIBIT manufactured by Cobra Tools, P.O. Box 356, Dewitt, Neb. 68341. The UNIBIT is formed with a rigid cylindrical body having a groove formed partially therethrough. A carbide cutting edge is mounted to one edge of the groove for cutting material from the workpiece. The cutting edge extends slightly above a circular front face of the cylindrical body in order to lead the device in the cutting operation. The rigid form of the cylindrical body provides substantially greater support than the serrated tooth cutting device discussed above.
The single edge cutting device has several limitations. For example, the cutting device requires many manufacturing operations to form the cylindrical body and requires cutting a groove therethrough. The body is formed of an appropriately strong material such as some type of steel. The cutting edge attached in the groove is formed of different material, usually a material having desired cutting characteristics such as carbide. A notch must be precisely formed in the groove to receive the separate cutting edge. After the carbide cutting edge is attached to the notch burrs or irregularities must be removed from the joint between the cutting tip and the notch to avoid forming an eccentric hole when the cutting device is used. Additionally, it is possible that this carbide tip requires sharpening after these operations in order to provide an efficient cutting edge.
Not only are there problems in the manufacturing of the single cutting edge device, but such a device also encounters problems in operation. The single cutting edge tends to be unbalanced to some degree since the single cutting edge is inherently unstable. A lack of balance of the single cutting edge device results in a degree of wobble increasing the inaccuracy of the hole formed thereby as well as increasing the friction and subsequently the wear and the effort required in cutting a hole.